A government watchdog has expressed "grave concerns" over plans to select a new head of the Northern Ireland Civil Service without an external competition for the post.

The Civil Service Commission spoke out after the current office holder, Sir Bruce Robinson, announced he was to seek a replacement from within the organisation's ranks.

Commissioners have sought an early meeting with him and said they believe the post, which commands a £141,800 starting salary, should be filled through an open recruitment process.

The intervention comes only days after Sir Bruce formally notified the commissioners of his plans on July 29.

A Stormont source said the post is being recruited from within the civil service because of the nature of the top job, with internal appointments common in similar jurisdictions.

Commission chair Brenda McLaughlin said: "While the decision to hold an internal competition is a matter for the NICS (Northern Ireland Civil Service), our view is that the merit principle is best served through fair and open competition."

In a statement, the watchdog group said it "expressed grave concern about the proposed arrangements for the appointment of the next head of the NI Civil Service (NICS) by way of an internal competition".

Commissioners have a statutory duty to ensure that appointments to the NICS are made on merit, on the basis of fair and open competition. They are also to publish a recruitment code.

Last October, Sir Bruce informed First Minister Peter Robinson and Deputy First Minister Martin McGuinness that he was to retire as NICS head. He acts as the key government adviser to the two political leaders.

The politicians will choose a replacement from a panel supplied to them at the end of the selection process, which is expected to finish around the end of September.